1.) Don't you need third party preset as witness in order to validate such private contract?
2.) Some state prohibits wage garnishment. Even court rules favorable verdict to him, it doesn't necessary mean he will have the enforcement to collect the debt except hand the case to a collection agency.
3.) At 40K, i think it's beyond small claims court amount, he needs to hire a lawyer to handle the case for him, well, that's another expense.